This invention relates generally to radio frequency circuits and more particularly to radio frequency amplifier circuits.
As is known in the art, in order to provide radio frequency circuits which operate over relatively large and high operating frequency bandwidths, it is common practice to use as an active element in such amplifiers a so-called metal semiconductor field effect transistor (MESFET). Typically, such a MESFET is comprised of Group III-V materials, such as gallium arsenide. One problem associated with the use of a gallium arsenide MESFET, in particular in low noise, wide-band amplifier applications, is that the gallium arsenide MESFET has relatively high levels of nonlinearity. The high levels of non-linearity inherent in the MESFET produce undesired distortion in a amplified signal provided from the amplifier.
The transfer function of a GaAs FET operating as an amplifier can be expressed as: EQU E.sub.0 =K.sub.1 E.sub.1 +K.sub.2 E.sub.i.sup.2 +K.sub.3 E.sub.i.sup.3 . . . K.sub.i E.sub.i.sup.3 Equation ( 1)
where E.sub.i and E.sub.0 are the input and output signal amplitudes, K is the linear gain coefficient and K.sub.i 's for i&gt;1 are the ith-order nonlinear coefficients. For a two-tone input signal, E.sub.i =A (cos .omega..sub.1 t+cos .omega..sub.2 t), the output amplitude of the signal becomes EQU E.sub.0 =DC+1st-order terms+2nd-order terms+3rd-order terms+. . . Equation (2)
where EQU 2nd-order terms=K.sub.2 A.sup.2 (cos(.omega..sub.i +.omega..sub.2)t+cos(.omega..sub.1 -.omega..sub.2)t)+1/2K.sub.2 A.sup.2 (cos(2.omega..sub.1 t)+cos(2.omega..sub.2 t)) Equation (3)
From equation 3 it may be seen that both the second harmonics (2.omega..sub.i) terms and the second-order intermodulation products (.omega..sub.i .+-..omega..sub.2) terms are related to the second-order nonlinear coefficient, K.sub.2. Generally, the second-order intermodulation products are 6 dB higher than the second harmonics.
It may often occur that for a wide-band system second order products, that is frequency components comprised of sum and/or difference frequencies of frequency components in the input signal, may exist within the operating band of the amplifier, are thus amplified in the system. This is undesirable because they may cause ambiguities in the wideband system, as well as contribute to a reduction in power added efficiency of the amplifier.